This invention relates to a sintered ceramic articles and a method for production thereof excelling in resistance to oxidation at elevated temperatures and enjoying good surface conditions.
Since sintered ceramic articles formed preponderantly of Si.sub.3 N.sub.4 excel in heat resistance and resistance to thermal shock, the feasibility of such articles in applications to structural parts such as gas turbine blades and nozzles which are required to offer high strength at elevated temperatures is being studied.
It is known that these sintered Si.sub.3 N.sub.4 ceramic articles have their strength lowered by oxidation at elevated temperatures exceeding 1200.degree. C. Thus, the desirability of improving these articles in resistance to oxidation is finding growing recognition. Davidge et.al. discloses (U.S. Pat. No. 3,824,120) a method of increasing the strength of a product containing porous Si.sub.3 N.sub.4 to be formed SiO.sub.2 to internal pores and on external surface by oxidizing at about 1000.degree. C. The method of this invention is effective for the porous products. However, this invention has no aptitude for improvement of Si.sub.3 N.sub.4 products which have a fine density concerning only the surface. Namely, since a degradation by oxidation and forming of SiO.sub.2 proceed simultaneously at elevated temperature, it is difficult to set a condition for oxidation. Rather, it causes decreased strength of the products.
Fredriksson discloses (U.S. Pat. No. 4,187,344) that an improvement in resistance to oxidation at elevated temperatures by coating a slurry of Si.sub.3 N.sub.4 or Si.sub.2 OH.sub.2 etc. to porous refractory article, heating at about 1000.degree. C., increasing strength of internal pores and external surface. However, this invention relates to slurry coating and it is difficult to apply a slurry; and its effectiveness seems to be not enough.
Sarin et.al. discloses (U.S. Pat. No. 4,406,668) that cutting tools comprising a silicon nitride substrate consists of a first phase of silicon nitride and a second phase of silicon nitride and metal oxide (Sio.sub.2, Al.sub.2 O.sub.3 etc.), and additionally coated metal nitride (TiN etc.). The product of invention is to be anticipated as a fine sintered ceramic article, but it has no aptitude for a material such as turbine blade. Coating requires special instruments such as CVD.
These sintered ceramic articles are generally desired, for the purpose of decreasing the cost of sintering, to be producted by the procedure of placing a plurality of shaped articles of Si.sub.3 N.sub.4 in a container made of Al.sub.2 O.sub.3 and sintering them in a furnace under an inactive gas atmosphere. This method of sintering however, has entailed a problem that during the course of sintering part of Si.sub.3 N.sub.4 is decomposed into Si or SiO.sub.2 and N.sub.2 and expelled out of the shaped articles, leaving behind coarsened surface.